This invention pertains to the field of digital imaging, and more particularly to a colorant reduction method used in the process of printing a digital image.
In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer may use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
Often when printing digital images, undesirable image artifacts may result when an excessive amount of colorant is placed in a small area on the page. These image artifacts degrade the image quality, and can result in an unacceptable print. In the case of an inkjet printer, some examples of these image artifacts include bleeding, cockling, banding, and noise. Bleeding is characterized by an undesirable mixing of colorants along a boundary between printed areas of different colorants. The mixing of the colorants results in poor edge sharpness, which degrades the image quality. Cockling is characterized by a warping or deformation of the receiver that can occur when printing excessive amounts of colorant. In severe cases, the receiver may warp to such an extent as to interfere with the mechanical motions of the printer, potentially causing damage to the printer. Banding refers to unexpected dark or light lines or streaks that appear running across the print, generally oriented along one of the axes of motion of the printer. Noise refers to undesired density or tonal variations that can give the print a grainy appearance, thus degrading the image quality. Although these artifacts are presented in the context of an inkjet printer, it is known to those skilled in the art that similar artifacts commonly exist with the other above mentioned printing technologies also.
State of the art inkjet printers designed for printing digital images sometimes utilize additional light density colorants to provide for improved image quality in the highlight regions of the image. In highlight regions, individual dots are routinely encountered, and if printed with a high density ink, these dots can be visible and objectionable to the observer. Using a lighter density ink in these highlight regions reduces the visibility of the individual dots so that the observer can no longer perceive them as distinct dots, thereby improving the image quality. However, such printers still require a high density ink to achieve acceptable density in shadow regions of the image. Thus, a typical combination of inks used in a high quality inkjet printer is the traditional cyan, magenta, yellow, and black (CMYK) inks plus additional light cyan and light magenta inks, indicated by lowercase xe2x80x9ccmxe2x80x9d, (i.e., CMYKcm). In these printers, the above mentioned problems associated with using excess colorant become worse, as the total amount of ink used is typically greater than for a standard CMYK printer.
In many inkjet printers, satisfactory density and color reproduction can generally be achieved without using the maximum possible amount of colorant. Therefore, using excessive colorant not only introduces the possibility of the above described image artifacts occurring, but is also a waste of colorant. This is disadvantageous, since the user will get fewer prints from a given quantity of colorant. It has been recognized in the art that the use of excessive colorant when printing a digital image needs to be avoided. Generally, the amount of colorant needed to cause image artifacts (and therefore be considered excessive) is receiver, colorant, and printer technology dependent. Many techniques of reducing the colorant amount are known for CMYK printers in which a halftoning process is used (typically inside a software printer driver program) to convert input digital image data into binary xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d states at each pixel. In such printers, the input image to the halftoning process is a higher bit precision image, typically 8 bits (or 256 levels) per pixel, per color.
U.S. Pat. No. 4,930,018 to Chan et al. teaches a method of reducing paper cockle and graininess of inkjet prints utilizing multiple inks with different dye loadings. In this method, a given grey level can be reproduced a variety of different ways, some of which will use more colorant than others. The different ways to reproduce a given grey level are rank ordered according to the total ink coverage, and a selection is made by iterating through the order until one is found that satisfies a specified maximum coverage limit.
U.S. Pat. No. 5,515,479 to Klassen et al. teaches a method for reducing marking material (i.e., ink) coverage in a printing process by determining the ink coverage for each pixel in an image, determining if too much ink will be placed on the page in a given area, and reducing the amount of ink to an acceptable level by turning xe2x80x9coffxe2x80x9d a fraction of the pixels in the given area. The determination of which pixels to turn off is made by using a processing order through each area which tends to randomize the turn off effect. While this method successfully reduces the amount of ink placed on the page in a given area, it can introduce pattern noise into the image because of the processing order method of selecting which pixels to turn off. Also, the pixels that are turned off in each color separation are not correlated, which can lead to a grainy appearance to an image region that should appear otherwise uniform.
U.S. Pat. No. 5,563,985 to Klassen addresses the problem of pattern noise by selecting which pixels to turn off in response to a random number function. While this method successfully eliminates pattern noise that can be generated in a given area, it can introduce random noise into the image because the selection of which pixels to turn off is determined by a random process. While this may be visually less objectionable than pattern noise, it is still not optimal.
U.S. Pat. No. 5,012,257 to Lowe et al. describes a xe2x80x9csuperpixelxe2x80x9d printing strategy to reduce bleed across color field boundaries. This strategy limits printing to no more than two drops of ink per cell or pixel, and no more than a total of three drops per superpixel, where a superpixel consists of a 2xc3x972 array of pixel cells. This strategy controls bleed, but at a penalty in terms of color and spatial resolution.
The above mentioned references teach methods of reducing artifacts associated with excessive colorant usage by utilizing methods that operate on the digital data after halftoning. That is, the above techniques operate primarily on bitmaps of binary image data where each pixel is represented by a code value of 0 (xe2x80x9coffxe2x80x9d, meaning no colorant), or 1 (xe2x80x9conxe2x80x9d, meaning fall colorant). At this point in the imaging chain of a digital printer, much image information has been lost due to the halftoning process, and accurately controlling the total colorant amount becomes more costly and less accurate relative to a pre-halftoning algorithm. U.S. Pat. No. 5,633,662 to Allen et al. teaches a method of reducing colorant using a pre-halftoning algorithm that operates on higher bit precision data (typically 256 levels, or 8 bits per pixel, per color).
U.S. Pat. No. 5,872,896 to Li et al. teaches a pre-halftone ink limiting algorithm in which pixels that exceed the total ink limit are depleted to values that are generally substantially less than the total ink limit. The reason for this is to prevent a many to one mapping that can occur if all pixels greater than the total ink limit are mapped to the total ink limit. This is a similar concept to what is done in color gamut mapping, where colors outside the gamut are mapped to colors substantially inside the gamut boundary to make room for other colors that are further outside the gamut.
While the above mentioned references may provide acceptable ink limiting for inkjet printers with CMYK colorants, they are not optimal solutions for a printer with CMYKcm colorants.
Thus, there is a need for a colorant reduction algorithm which can be applied to a multicolorant printer utilizing both dark and light density colorants to provide for high quality images.
It is an object of the present invention to provide a method for printing high quality digital images that are free of the above described artifacts associated with using excessive amounts of colorant.
It is a further object of the present invention to provide a method for reducing the amount of colorant used to print an image on a multicolorant printer in which dark and light density colorants are used.
Yet another object of the present invention is to provide a method for reducing the amount of colorant used to print an image on a multicolorant printer in which dark and light density colorants are used such that the perceived color is substantially unchanged.
These objects are achieved by a method for a method for modifying an input image having an (x,y) array of pixels suitable for printing on a digital printer having two or more colorants, wherein at least two of the colorants are similar having substantially the same color but different densities, and wherein each pixel has input code values representing input colorant amounts of said two or more colorants, to form an output image with pixels representing modified colorant amounts subject to a total colorant amount limit, comprising the steps of:
a) determining a total input colorant amount for a pixel responsive to the input colorant amounts of said two or more colorants;
b) determining modified colorant amounts for the similar colorants responsive to the input colorant amounts, the total input colorant amount, and the total colorant amount limit such that a first colorant amount is removed from a lower density similar colorant and a second colorant amount less than the first colorant amount is added to a higher density similar colorant; and
c) repeating steps a) and b) for each pixel in the input image.
The present invention has an advantage over the prior art in that it provides for reducing the amount of colorant used to print a digital image on an inkjet printer having light and dark density colorants. Another advantage of the present invention is that the perceived color of the digital image is substantially preserved in the colorant reduction process wherever possible. Yet another advantage of the present invention is that the total amount of colorant is reduced without increasing the noise or graininess of the image. Still another advantage of the present invention is that the total amount of colorant is more accurately controlled relative to many of the prior art methods, providing for improved control over image artifacts associated with using excess colorant.